The marine bryozoan Bugula neritina has been found to contain a unique series of closely related macrocyclic (22-membered) lactones known as bryostatins. Because of their very selective antineoplastic and cytostatic activity, potent influence on protein kinase C biochemical pathways, anti-tumor promoter effects and stimulation of bone marrow progenitor cells to form colonies (GM-CSF activity), bryostatin 1 has been selected for clinical evaluation. Discovery and study of a bryostatin biosynthetic precursor or degradation products has been considered necessary to gain further mechanistic and structure/activity insights. The isolation and structural elucidation of the first such substance, herein designated "neristatin 1", is described herein.